1. Field of the Invention
The present invention relates to a method and apparatus for configuring a, quantum mechanical state required for quantum information processing, quantum communication, or quantum precision measurements, as well as a communication method and apparatus using such a method and apparatus.
2. Related Background Art
The fields of quantum computations (see R. P. Feynman, "Feynman Lectures on Computation," Addison-Wesley (1996)) and quantum information theories are advancing rapidly. In these fields, superposition, interference, and an entangled state, which are the basic nature of quantum mechanics, are ingeniously utilized.
In the field of quantum computations, since the publication of Shor's algorithm concerning factorization (see P. W. Shor, "Polynomial-Time Algorithms for Prime Factorization and Discrete Logarithms on a Quantum Computer," LANL quantum physics archive quant-ph/9508027. Similar contents are found in SIAM J. Computing 26 (1997), 1484. In addition, the first document is P. W. Shor, "Algorithms for quantum computation: Discrete logarithms and factoring," in Proceedings of the 35th Annual Symposium on Foundations of Computer Science (ed. S. Goldwasser) 124-134 (IEEE Computer Society, Los Alamitos, Calif., 1994). A detailed description is found in Artur Ekert and Richard Jozsa, "Quantum computation and Shor's factoring algorithm," Rev. Mod. Phys. 68, 733 (1996)) and Grover's algorithm concerning the search problem (L. K. Grover, "A fast quantum mechanical algorithm for database search," LANL quantum physics archive quant-ph/9605043. Almost similar contents are found in L. K. Grover, "Quantum Mechanics Helps in Searching for a Needle in a Haystack," Phys. Rev. Lett. 79, 325 (1997)), many researchers have been proposing methods for implementing quantum computations and developing new quantum algorithms.
On the other hand, in the field of quantum information theories, the entangled state has been known to play an important role due to its unlikelihood to be affected by decoherence (see C. H. Bennett, C. A. Fuchs, and J. A. Smolin, "Entanglement-Enhanced Classical Communication on a Noisy Quantum Channel," Quantum Communication, Computing, and Measurement, edited by Hirota et al., Plenum Press, New York, p. 79 (1997)).
Furthermore, as an application of these results, a method for overcoming the quantum shot noise limit using (n) two-level entangled states has been established through experiments on Ramsey spectroscopy (see D. J. Wineland, J. J. Bollinger, W. M. Itano, F. L. Moore, and D. J. Heizen, "Spin squeezing and reduced quantum noise in spectroscopy," Phys. Rev. A 46}, R6797 (1992) or D. J. Wineland, J. J. Bollinger, W. M. Itano, and D. J. Heizen, "Squeezed atomic states and projection noise in spectroscopy," Phys. Rev. A 50, 67 (1994)). Despite the lack of discussion of the two-level Ramsey spectroscopy, a similar concept is described in M. Kitagawa and M. Ueda, "Nonlinear-Interferometric Generation of Number-Phase-Correlated Fermion States," Phys. Rev Lett. 67, 1852 (1991).
If decoherence in the system caused by the environment is negligible, the maximally entangled state serves to improve the accuracy in measuring the frequency of an energy spectrum.
In this case, the fluctuation of the frequency decreases by 1/n. With decoherence in the system considered, however, the resolution achieved by the maximally entangled state is only equivalent to that achieved by an uncorrelated system.
In addition, the use of a partly entangled state having a high symmetry has been proposed in S. F. Huelga, C. Macchiavello, T. Pellizzari, A. K. Ekert, M. B. Plenio, J. I. Cirac, "Improvement of Frequency Standards with Quantum Entanglement," Phys. Rev. Lett. 79, 3865 (1997).
If optimal parameters (coefficients of basic vectors) can be selected beforehand, this method can provide a higher resolution than the maximally entangled or uncorrelated state.
The partly entangled state having a high symmetry is given by the following equation: ##EQU1##
where (n) represents the number of qubits that are twolevel particles constituting a state, and .right brkt-top.n/2.right brkt-bot. represents a maximum integer not more than n/2. {a.sub.k } is a real number wherein, for example, an optimal combination of values are assumed to be provided beforehand so as to provide a high resolution in the Ramsey spectroscopy. In this case, {a.sub.k } may be a constant.
.vertline.k&gt;.sub.s is a superposition of states in which (k) or (n-k) qubits are excited, wherein the superposition is established using an equal weight. For example, .vertline..psi..sub.4 &gt; is given by the following equation. ##EQU2##
These states have symmetry such as that described below.
Invariable despite the substitution of any two qubits PA0 Invariable despite the simultaneous inversion of {.vertline.0&gt;, .vertline.1&gt;} for each qubit.
To conduct experiments on the Ramsey spectroscopy using a partly entangled state having a high symmetry, a target entangled state must be provided as soon as possible as an initial system state before decoherence may occur. Thus, all actual physical systems have a decoherence time, and quantum mechanical operations must be performed within this time. This is a problem in not only quantum precision measurements but also quantum communication and general quantum computations.
In addition, to configure the target entangled state from a specified state, an operation using basic quantum gates must be performed out many times. Minimizing this number of times leads to the reduction of the time required to provide the target entangled state.
In addition, to configure a quantum gate network for obtaining the target entangled state, a conventional computer must be used to determine in advance which qubits will be controlled by the basic quantum gates, the order in which the basic quantum gates will be used, and a rotation parameter for unitary rotations. The amount of these computations is desirably reduced down to an actually feasible level.